Enzymes and Their Functions

Enzymes and Enzyme Action

• Enzymes are biological catalysts
• Increase the rate of a reaction
• Remain unchanged during the reaction
• Allow for faster conversion of reactants to products

Enzymes and Active Sites

• Active site is where the substrate fits for reactions to occur
• Substrate (e.g., lactose) is held in place by hydrogen bonds with amino acid R groups of the enzyme (e.g., lactase)
• Amino acids within the active site are crucial

Enzyme-Substrate Complex

• Active site of lactase is flexible to accommodate the substrate
• The reaction complex is complete when the disaccharide is hydrolyzed
• Monosaccharide products are released from the enzyme
• The enzyme is ready to bind another lactose molecule

Models of Enzyme Action

• Lock-and-key model: One enzyme fits one substrate
• Induced-fit model: More dynamic, flexible active site adapts to substrate shape to assist product formation and lower the activation energy, this is a more modern view of enzyme action.

Learning Check

• Active site is a section of the enzyme.
• In the induced-fit model, the enzyme's shape changes to fit the substrate.

Names of Enzymes

• Enzyme names typically end in -ase
• Enzyme names may indicate the reacting substance (e.g., sucrase for sucrose)
• Enzyme names may reflect the function (e.g., oxidases for oxidation)
• Common names for digestive enzymes exist (e.g., pepsin and trypsin)

Factors Affecting Enzyme Activity

• Enzyme activity is the speed of reaction catalysis
• Activity is heavily influenced by factors like temperature, pH and enzyme and substrate concentration

Enzyme Activity and Temperature

• Enzymes show peak activity at an optimum temperature.
• Low temperatures result in low enzymatic activity
• High temperatures cause denaturation and loss of activity

Enzyme Activity and pH

• Enzymes function best at an optimal pH.
• pH variations affect amino acid charges, impacting enzyme effectiveness
• Extreme pH values can disrupt the enzyme's tertiary structure and cause loss of function

Optimum pH values

• Enzymes in different organs (e.g., stomach, small intestine, liver) operate at specific pH values.
• Specific enzymes in the table are associated with their optimal pH values and their locations.

Enzyme Concentration

• Increasing enzyme concentration (at constant substrate concentration) increases the reaction rate
• More substrate binding to the enzyme occurs

Substrate Concentration

• Increasing substrate concentration (at constant enzyme concentration) increases the reaction rate.
• Enzyme eventually becomes saturated with substrate, reaching maximum activity

Feedback Control: Metabolism When Needed

• End product of a series of reactions acts as a negative regulator for the initial enzyme, stopping the reaction chain
• This process stops until the end product is no longer present to bind to the initial enzyme, then the reaction process restarts.

Zymogens: Proenzymes

• Large, inactive enzyme precursors are produced, activated when needed
• Examples include pepsinogen, trypsinogen, and others.
• The enzymes are produced in one location and then activated in another.

Enzyme Activity Regulation

• Kinase activates an inactive enzyme by adding a phosphate group (phosphorylation)
• Phosphatase activates an inactive enzyme by removing a phosphate group

Enzyme Cofactors

• Simple enzymes: Contain only protein
• Many enzymes require cofactors.
• Cofactors are small molecules such as metal ions or coenzymes.
• Coenzymes are organic molecules, often vitamins, that assist in enzyme function
• Table 20.9 shows enzymes that require metal ion cofactors.

Vitamins and Coenzymes

• Vitamins are organic molecules essential for health and growth
• Vitamins are needed in trace amounts

Water-Soluble Vitamins (B, C, H)

• Must be regularly replenished
• Not stored in the body, excess excreted
• Function as cofactors for enzymes
• Can be destroyed by heat (boiling)

Fat-Soluble Vitamins (A, D, E, K)

• Stored in the body
• Involved in vision, bone formation, antioxidants, and blood clotting.
• Not coenzymes for catalytic reactions

This quiz explores the key concepts related to enzymes, including their role as biological catalysts, structure, and the mechanisms of enzyme action. It covers important topics such as enzyme-substrate complexes and models of enzyme action. Test your knowledge on how enzymes facilitate chemical reactions and the significance of active sites.